A known air-conditioning apparatus used in, for example, a multi-air-conditioning apparatus for a building causes a refrigerant to circulate between an outdoor unit being a heat source device arranged, for example, outside a structure and an indoor unit arranged inside a room in the structure. Air-conditioned space where the indoor unit is arranged is cooled or heated by air heated or cooled by the refrigerant rejecting or removing the heat. One example refrigerant frequently used in such an air-conditioning apparatus may be a hydrofluorocarbon (HFC) refrigerant. There is also an example that uses a natural refrigerant, such as carbon dioxide (CO2).
One existing air-conditioning apparatus used in, for example, a multi-air-conditioning apparatus for a building is the one in which a refrigerant is circulated between an outdoor unit and a relay unit, a heat medium is circulated between the relay unit and an indoor unit, and air-conditioned space where the indoor unit is arranged is cooled or heated by heat exchange between the refrigerant and the heat medium in a heat exchanger in the relay unit (see, for example, Patent Literature 1).